1. Field of the Invention
This invention relates to seismic prospecting, and more particularly, to a new method and apparatus for detecting seismic signals and generating signals for subsequent seismic analysis.
2. Prior Art
In seismic prospecting, seismic waves are generated in an elastic medium such as land or water, and the reflected seismic waves are detected with sensors such as hydrophones, geophones and accelerometers. A geophone is a velocity sensitive detector which must be well coupled to the earth in order to sense meaningful seismic information. Geophones are used primarily on land because of this coupling requirement. A hydrophone is a pressure sensitive detector which must be fully submersed in water or liquid in order to work properly. Hydrophones used in commercial marine seismic surveys are designed to be insensitive to acceleration forces. To increase sensitivity to pressure signals, pairs of piezo-electric ceramic wafers are secured to pliable diaphragms. Pressure variations flex the wafers which generate a signal proportional to the pressure variations. The wafers are also subject to acceleration forces but, because the mass of the wafers is very small, the acceleration component of the total output signal is comparable to the ambient noise, and is inseparable therefrom. Since the acceleration signals cannot be separated from the noise signals, their combination tends to increase the noise contamination of the desired pressure signal. For this reason, the outputs of the individual wafers of the pair are electrically connected so as to cancel the acceleration signal and to reinforce pressure signals. Accelerometers sense particle acceleration and are also used primarily on land in order to sense pertinent seismic data. Historically, seismic surveying was performed primarily on land using geophones and/or accelerometers. Later, seismic prospecting moved offshore using hydrophones. However, areas such as swamps, marshes, rivers, bays and the like must also be surveyed in which the use of only one type of such seismic sensors does not always yield meaningful information. In such transitional areas, it is impractical to attempt to use more than one type of sensor in a particular survey in order to use, for example, hydrophones in water and geophones on land because the actual limits of land and water are not always easily defined beforehand.
Prior devices, such as shown in U.S. Pat. No. 3,921,755, have used both hydrophones and geophones connected to the same cable. Their output signals are simultaneously but separately recorded for later processing and comparison. A similar system is disclosed in U.S. Pat. No. 3,629,801. In other prior devices, hydrophones and geophones or accelerometers have been combined within a single housing to provide one type of seismic sensor package for these purposes.
Hydrophones have been combined with geophones as shown in U.S. Pat. Nos. 2,740,945; 2,846,662; and 3,332,057, in order to provide directional sensitivity: That is, to discriminate between desired upwardly-directed reflections from subterranean earth layers, and downwardly traveling surface waves or direct arrivals. Such devices are not satisfactory for use in transition areas because the output signals from velocity sensors (geophones) and hydrophones (pressure sensors) are 90.degree. out of phase with each other and therefore they cannot be combined meaningfully without special processing.
Hydrophones have been combined with accelerometers in U.S. Pat. Nos. 3,158,831; 3,281,763 and 3,311,873. In the first two patents and, by reference in the third patent, the accelerometer output signals are integrated in order to produce a signal proportional to particle velocity. These devices are not suitable for use in transition zones for the reasons discussed above.
U.S. Pat. Nos. 3,320,580 and 3,320,582 disclose a combination sensor wherein a single pair of piezo-electric crystal elements is employed to determine particle displacement, velocity, acceleration, pressure and pressure gradient. The basic output signal is proportional to the combined forces due to pressure and acceleration. Acceleration signals are separated from pressure signals by addition and subtraction circuitry. The remaining parameters listed above are derived by additional signal processing. The device is basically an acceleration-sensitive hydrophone. In the seismic exploration industry, as discussed previously, commercially available hydrophones are constructed so as to be insensitive to acceleration forces to prevent the acceleration signals from combining with ambient noise and further contaminating the pressure signals. Accordingly the acceleration-sensitive device disclosed in the above two patents is unsuited for marine or transition-zone use.
Throughout this application, where required, the seismic industry standard convention with respect to seismic sensor polarities will be used. The positive terminal of the device is considered to be the one which is negative with respect to the other terminal for a pressure increase or an upwardly-directed motion or acceleration of the earth.
Applicant has discovered that the theoretical assumptions taught by the prior art are in error and that the output signals of pressure sensitive and acceleration sensitive sensors may be combined directly to produce a meaningful signal for seismic analysis.